Color electrophotographic printers provide full color images by building up and sequentially transferring individual color separation toner images in registration onto a receiver and fusing the toner and receiver. Specific color outcomes are achieved in such printers because controlled ratios of differently colored toners are applied in combination to create appearance of a desired color at specific locations on a receiver. Similarly, as is described in U.S. Patent Publication Number: US20090286177A1, entitled “Adjustable Gloss Document Printing” different toners such as high viscosity toners can be used in combination with lower viscosity toners to allow a user to obtain an adjustable gloss. The gloss is made adjustable by controlling the ratio of the two types of toner in the combination.
It will be appreciated that many other desirable printing outcomes can be achieved using ratio controlled combinations of toners. However, a central limitation on the use of multiple different toner types in electrophotographic printers and methods is that electrophotographic printing modules of the type that form the individual toner images can be large, complicated and expensive. Further, it is difficult to ensure registration of the printing modules with the transfer systems and receivers in a digital printer and such difficulties increase with each additional printing module that is to be incorporated into a printer.
Accordingly, printers are typically designed to provide a limited number of such electrophotographic printing modules. For example, the Nexpress 2100 and subsequent models provide a tandem arrangement of five printing modules. During printing of color image four of these tandem printing modules apply different ones of four toners, each supplying one of the four primary subtractive colors, while a fifth printing module is used to apply custom colors, clear overcoats and other different types of toner to the formed color toner image. The fifth printing module can be used add toners to the color toner image in precise ratios relative to the toners that have previously been applied. While this can be done in a highly effective and commercially viable manner, there remains a need in the art for methods that enable toner images to be formed for use in making an electrophotographic print that include a greater number of different toners than the limited number that are currently available and that can provide such toners in controlled registration and in a manner that can be adjusted on a picture element by picture element basis.
In one alternative, U.S. Pat. No. 5,926,679, issued to May, et al., discloses that a clear (non-marking) toner layer can be laid down on a photoconductive member (e.g., imaging cylinder) prior to forming a marking particle toner image thereon, and that a clear toner layer can be laid down as a last layer on top of a marking particle toner image prior to transfer of the image to an intermediate transfer member (e.g., blanket cylinder). It is also disclosed that a clear toner layer can be laid down on a blanket cylinder prior to transferring a marking particle toner image from a photoconductive member. In one aspect of this patent, a non-imagewise clear toner layer is bias-developed on to an intermediate transfer member using a uniform charger and a non-marking toner development station. A first monocolor toner image corresponding to one of the marking toners is transferred to the ITM (on top of the clear toner) from a primary imaging member which may be a roller or a web but is preferably a roller. Subsequently, a second monocolor toner image corresponding to another of the marking toners is transferred to the ITM (on top of and in registration with the first toner image) and so forth until a completed multicolor image stack has been transferred on top of the clear toner on the ITM. The ITM is then positioned at a sintering exposure station; where a sintering radiation is turned on to sinter the toner image for a predetermined length of time.
However, while this approach can be effective and can provide a commercially viable solution, this approach requires an additional transfer step for each toner that is applied which, in turn, reduces machine productivity.
Accordingly, what is needed in the art are printers and printing methods that enable an increase in the opportunities to use the features of ratio controlled combinations of toners without compromising the efficiency and the accuracy of registration with which each of the toners can be provided.